Light emitting apparatus, method for driving the light emitting apparatus, and display apparatus including the light emitting apparatus

ABSTRACT

A light emitting apparatus comprises a light emitting section for emitting light, a color of the light being changed with a value of a driving current, and a driving section for driving the light emitting section so that the light emitting section emits light having a desired color and a desired intensity, by generating the driving current based on a signal designating the desired color and a signal designating the desired intensity and by applying the driving current to the light emitting section.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light emitting apparatus and amethod for driving the light emitting LED apparatus. The presentinvention also relates to a display apparatus including the lightemitting apparatus.

[0003] 2. Description of the Related Art

[0004]FIG. 20 illustrates a conventional LED driving apparatus. In theconventional LED driving apparatus, a driving current i corresponding toan output establishment value p is applied from a power source 191 to anLED device 192 to drive the LED device 192. The emission intensity ofthe LED device 192 is controlled by changing the value of the drivingcurrent i. In general, the color of light emitted from conventional LEDdevices does not depend on the value of a driving current. Thus, theemission intensity of conventional LED devices is changed by control ofthe driving current value while maintaining the color of light emittedfrom the LED devices.

[0005] The emission intensity of conventional LED devices can be solelychanged by control of the driving current value, since the light colorof the conventional LED devices is not affected by a change in thedriving current value. However, there is another type of LED devicehaving a light color which is altered by a change in the driving currentvalue. When such an LED device is driven by a driving apparatus as shownin FIG. 20, the light color of the LED device is changed along with theemission intensity control thereof. Thus, a desired emission intensitycontrol cannot be conducted.

[0006] Conventional AlGaAs semiconductor light emitting devices have aphenomenon in which the wavelength of light emitted therefrom becomeslonger as a driving current is Increased (red shift phenomenon). The redshift phenomenon occurs as follows. The light emitting device is heatedby supplied power, and the temperature of the device is proportional tothe average supplied power. The increased temperature causes the bandgapof the active layer to be reduced, so that the wavelength of lightemitted from the device becomes longer (red shift). In other words, theemission intensity and emission wavelength of the light emitting devicehaving the red shift phenomenon are determined by the average powerapplied to the device. It is not possible to separately control theemission wavelength and emission power of the AlGaAs semiconductor lightemitting device using two parameters (peak value and average power) of adriving current pulse.

SUMMARY OF THE INVENTION

[0007] According to one aspect of the present invention, a lightemitting apparatus comprises: a light emitting section for emittinglight, a color of the light being blue shifted with a value of a drivingcurrent; and a driving section for driving the light emitting section sothat the light emitting section emits light having a desired color and adesired intensity, by generating the driving current based on a signaldesignating the desired color and a signal designating the desiredintensity and by applying the driving current to the light emittingsection.

[0008] In one embodiment of this invention, the driving sectioncomprises: a converting section for converting the color-designatingsignal to a signal designating a current value in accordance withcurrent versus emission wavelength characteristics of the light emittingsection; a calculating section for calculating a signal designating aduty based on the intensity-designating signal and the currentvalue-designating signal, in such a manner that the product of a currentvalue designated by the current value-designating signal and a dutycorresponds to the intensity designated by the intensity-designatingsignal; and a section for generating, as the driving current, a pulsecurrent having the current value and the duty in accordance with therespective current value-designating signal and duty-designating signal.

[0009] In one embodiment of this invention, the light emitting sectionis an LED device.

[0010] In one embodiment of this invention, the light emitting sectionto an LED bullet comprising: an LED device; and a fluorescent excited bylight emitted by the LED device to emit light.

[0011] In one embodiment of this invention, when the driving currentvalue is changed, a change in an emission wavelength of the LED bulletis larger than a change in an emission wavelength of the LED device.

[0012] In one embodiment of this invention, the light emitting apparatuscomprises: a plurality of the light emitting sections; and a pluralityof the driving sections respectively corresponding to the plurality ofthe light emitting sections. A color and intensity of light emitted byeach of the plurality of the light emitting sections is controlled by acorresponding one of the plurality of the driving sections.

[0013] In one embodiment of this invention, the color-designating signalis a color signal for designating a color of light to be emitted by theLED device, and the intensity-designating signal is an intensity signalfor designating an intensity of the light to be emitted by the LEDdevice.

[0014] In one embodiment of this invention, the color-designating signalis a chromaticity signal for designating a color of light to be emittedby the LED bullet, and the intensity-designating signal is a luminancesignal for designating an intensity of the light to be emitted by theLED bullet.

[0015] In one embodiment of this invention, the LED device comprises: anInGaN active layer; and an AlGaN layer having two layers, provided onthe InGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

[0016] In one embodiment of this invention, the LED device comprises: anInGaN active layer; and an AlGaN layer having two layers, provided onthe InGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

[0017] According to another aspect of the present invention, a methodfor driving a light emitting apparatus is provided. The apparatuscomprises: a light emitting section for emitting light, a color of thelight being blue shifted with a value of a driving current; and adriving section for driving the light emitting section. The methodcomprises the steps of: receiving a signal designating a desired colorand a signal designating a desired intensity; generating the drivingcurrent based on the color-designating signal and theintensity-designating signal; and applying the driving current to thelight emitting section so that the light emitting section emits lighthaving the desired color and the desired intensity.

[0018] In one embodiment of this invention, the method further comprisesthe steps of: converting the color-designating signal to a signaldesignating a current value in accordance with current versus emissionwavelength characteristics of the light emitting section; calculating asignal designating a duty based on the intensity-designating signal andthe current value-designating signal, in such a manner that the productof a current value designated by the current value-designating signaland the duty corresponds to the intensity designated by theintensity-designating signal; and generating as the driving current apulse current having the current value and the duty in accordance withthe respective current value-designating signal and duty-designatingsignal.

[0019] In one embodiment of this invention, the light emitting sectionis an LED device.

[0020] In one embodiment of this invention, the light emitting sectionis an LED bullet comprising: an LED device; and a fluorescent excited bylight emitted by the LED device to emit light.

[0021] In one embodiment of this invention, when the driving currentvalue is changed, a change in an emission wavelength of the LED bulletis larger than a change In an emission wavelength of the LED device.

[0022] In one embodiment of this invention, the light emitting apparatuscomprises: a plurality of the light emitting sections and a plurality ofthe driving sections respectively corresponding to the plurality of thelight emitting sections. A color and intensity of light emitted by eachof the plurality of the light emitting sections is controlled by acorresponding one of the plurality of the driving sections.

[0023] In one embodiment of this invention, the color-designating signalis a color signal for designating a color of light to be emitted by theLED device, and the intensity-designating signal is an intensity signalfor designating an intensity of the light to be emitted by the LEDdevice.

[0024] In one embodiment of this invention, the color-designating signalis a chromaticity signal for designating a color of light to be emittedby the LED bullet, and the intensity-designating signal is a luminancesignal for designating an intensity of the light to be emitted by theLED bullet.

[0025] In one embodiment of this invention, the LED device comprises: anInGaN active layer; and an AlGaN layer having two layers, provided onthe InGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

[0026] In one embodiment of this invention, the LED device comprises: anInGaN active layer; and an AlGaN layer having two layers, provided onthe InGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

[0027] In one embodiment of this invention, the light emitting sectionis an LED device, the LED device is driven by a pulse current having acurrent value and a duty, the current value is set to a valuecorresponding to an emission wavelength of the LED device, and the dutyis set to a value corresponding to an emission intensity of the LEDdevice.

[0028] In one embodiment of this invention, the current value is set toa value in a range in which a change in the emission wavelength of theLED device is about 6 nm, and the duty is changed in accordance with theemission intensity of the LED device.

[0029] In one embodiment of this invention, the current value iscontrolled so that the emission wavelength of the LED device is changed,and the duty is controlled so that the emission intensity of the LEDdevice is substantially maintained constant.

[0030] In one embodiment of this invention, the light emitting sectionis an LED bullet comprises: an LED device; and a fluorescent excited bylight emitted by the LED device to emit light. An emission wavelength ofthe LED device is changed by changing the driving current value so thata color of light emitted by the LED bullet is changed.

[0031] In one embodiment of this invention, the light emitting sectionis an LED bullet comprises: an LED device; and a fluorescent excited bylight emitted by the LED device to emit light. The LED device is drivenby a pulse current having a current value and a duty wherein the currentvalue is set to a value in accordance with current versus emissionwavelength characteristics of the LED bullet, and the duty is set to avalue in accordance with emission intensity characteristics of the LEDbullet.

[0032] According to another aspect of the present invention, a displayapparatus comprises: a plurality of light emitting sections provided ina plane, light emitted by each of the plurality of light emittingsections being blue shifted with a value of a driving current; and aplurality of driving sections each for driving a corresponding one ofthe plurality of light emitting sections so that the corresponding oneof the plurality of light emitting sections emits light having a desiredcolor and a desired intensity, by each generating the driving currentbased on a signal designating the desired color and a signal designatingthe desired intensity and by each applying the driving current to thecorresponding one of the plurality of light emitting sections.

[0033] In one embodiment of this invention, each of the plurality oflight emitting sections is an LED device.

[0034] In one embodiment of this invention, each of the plurality oflight emitting sections is an LED bullet comprising: an LED device; anda fluorescent excited by light emitted by the LED device to emit light.

[0035] According to another aspect of the present invention, an LEDbullet comprises: an LED device having a light color, the light colorbeing blue shifted with a change in a driving current; and a fluorescentexcited by light emitted by the LED device to emit light.

[0036] Thus, the invention described herein makes possible theadvantages of providing (1) a method for driving an LED device having alight color which is blue shifted with a driving current value, in sucha manner that the emission intensity of the LED device is changed whilemaintaining the light color of the LED device from being changed; (2) amethod for driving an LED device in which variation of the emissionintensity and light color of the LED device can be suppressed; (3) amethod for driving an LED device in which light emitted from the LEDdevice can be changed by utilizing a characteristic of the LED device inwhich the light color of the LED device is changed with a currentdriving value, (4) an apparatus for achieving the above-describedmethods; (5) an LED bullet including an LED device and a fluorescent andcapable of providing a wide range of variations in light color; and (6)a method for driving the LED bullet.

[0037] These and other advantages of the present invention will becomeapparent to those stilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic diagram Illustrating an LED apparatusaccording to Example 1 of the present invention.

[0039]FIG. 2 a schematic diagram illustrating a configuration of an LEDdevice according to Example 1 of the present invention.

[0040]FIG. 3 is a graph showing current versus emission wavelengthcharacteristics of the LED device of FIG. 2.

[0041]FIG. 4 is a graph showing an emission wavelength spectrum of theLED device of FIG. 2.

[0042]FIG. 5 is a graph showing the product of driving current and dutyversus emission power characteristics of the LED device of FIG. 2 drivenby the LED on-off circuit of the present invention shown in FIG. 1.

[0043]FIG. 6 is a schematic diagram illustrating a pulse signalgenerated by a pulse generator of the LED on-off circuit of the presentinvention shown in FIG. 1.

[0044]FIG. 7 is a schematic diagram illustrating an LED apparatusaccording to Example 3 of the present invention.

[0045]FIG. 8 is a graph showing the current versus emission powercharacteristics of an LED device driven by the LED on-off circuit of thepresent invention shown in FIG. 7.

[0046]FIG. 9 is a schematic diagram illustrating an LED bullet accordingto Example 5 of the present invention.

[0047]FIG. 10 is a graph showing the excitation spectrum and emissionspectrum of a fluorescent used in the LED bullet of Example S.

[0048]FIG. 11 is a CIE standard chromaticity diagram.

[0049]FIG. 12 is a graph showing the current versus emission wavelengthcharacteristics of an LED device employed in Example 6 of the presentinvention.

[0050]FIG. 13 is a UCS chromaticity diagram.

[0051]FIG. 14 is a graph showing the excitation spectrum and emissionspectrum of a fluorescent used in Example 6.

[0052]FIG. 15 is a graph showing luminance characteristics of an LEDbullet employed in Example 7 of the present invention.

[0053]FIG. 16 is a graph showing the u value versus currentcharacteristics of the LED bullet in Example 7.

[0054]FIG. 17 is a schematic diagram illustrating an LED on-off circuitemployed in Example 7.

[0055]FIG. 18 is a schematic diagram illustrating forms of generatedpulse signals.

[0056]FIG. 19 is a schematic diagram illustrating a display apparatusaccording to Example 10 of the present invention.

[0057]FIG. 20 is a schematic diagram of a conventional LED on-offapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0058] Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

[0059] The present invention provides a method for controlling an LEDdevice having an active layer which is an InGaN quantum well layer andan emission wavelength which is blue shifted (becomes shorter). In themethod of the present invention, the emission power and emissionwavelength of the LED device can be tuned to desired values by employinga pulse driving system. The inventors have focused on the fact that theblue shift in the LED device is determined depending on theinstantaneous amount of carriers in the active layer of the LED device.The emission wavelength of the LED device is controlled using the peakvalue of a pulse driving current. In the meantime, the emissionintensity of the LED device is controlled using the average power of thepulse driving current. Therefore, the pulse duration of the pulsedriving current is preferably as long as or longer than therecombination life span of a carrier in the active layer of the LEDdevice. This is because when the pulse duration is shorter than therecombination life span of a carrier in the active layer, the effectivecarrier density of the active layer is a value obtained by calculatingthe integral of carriers over the recombination life span of a carrier,so that the carrier amount of the active layer does not depend directlyon the peak value of each pulse. In general, the recombination life spanof a carrier in an LED device having the blue shift phenomenon of whichan active layer is made of InGaN is about 0.2 ns or more. The pulseduration of each pulse in the driving current is preferably about 0.2 nsor more, more preferably about 1 ns or more.

[0060] A major problem with the above-described blue shift is that thecolor tone of light emitted from the LED device is visually changed. Theobject of the present invention is to drive an LED device, where thetone of the emission intensity of the LED device is modulated by simplychanging a current amount, under conditions given by:

λ₁−λ₂≧6 nm and I ₁ <I ₂

[0061] where in the range from the minimum current value or more to themaximum current value or less, λ₁ is the longest emission wavelength andI₁ is the current value when the longest emission wavelength isobtained, and λ₂ is the shortest emission wavelength and I₂ is thecurrent value when the shortest emission wavelength is obtained.

[0062] For example, the present invention is applied to the case wherethe difference in emission wavelength between when the LED device isdriven by a direct current of about 0.1 mA and when the LED device isdriven by a direct current of about 30 mA is about 6 nm or moreConversely, when the LED device is controlled so that wavelength shiftis restricted to a predetermined level, the LED apparatus should bedesigned so that the wavelength shift falls within the range of about 6nm or less.

EXAMPLE 1

[0063] A light emitting apparatus according to Example 1 of the presentinvention includes: a light emitting section for emitting light in whicha color of the light is blue shifted with a value of a driving current:and a driving section for driving the light emitting section so that thelight emitting section emits light having a desired color and a desiredintensity, by generating the driving current based on a signaldesignating the desired color and a signal designating the desiredintensity and by applying the driving current to the light emittingsection. In description of Example 1, the light emitting section servesas an LED device, and the driving section serves as an LED on-offcircuit.

[0064]FIG. 1 illustrates an LED apparatus according to Example 1 of thepresent invention. The LED apparatus of Example 1 includes an LED on-offcircuit 1 for outputting an LED driving current R based on an intensitysignal p and a color signal c, and an LED device 104 having a lightcolor which is changed with a change in a driving current value.

[0065] The LED on-off circuit 1 includes: a color signal-current valuesignal converter 102 for converting the color signal c to a currentvalue signal i; a calculation processor 101 for calculating andoutputting a duty signal d designating a duty D, based on an intensitysignal p and the current value signal i; and a square wave pulsegenerator 103 for outputting a square wave pulse current R as the LEDdriving current based on the current value signal i and the duty signald.

[0066]FIG. 2 is a cross-sectional view illustrating a structure of theLED device 104 of Example 1. The LED device 104 is fabricated in thefollowing way. A GaN buffer layer (not shown), an n-type GaN layer 202,an In_(0.2)Ga_(0.8)N active layer 203, a p-type Al_(0.1)Ga_(0.9)Nevaporation prevention layer 204, a p-type Al_(0.1)Ga_(0.9)N upperevaporation prevention layer 205, and a p-type GaN contact layer 206 aresuccessively formed on a sapphire substrate 201. The resultingmulti-layer structure is dry-etched by reactive ion beam etching (RIBE)until a portion of the n-type GaN layer 202 is exposed. A nickel (Ni)film 207 is formed and patterned on part of the p-type contact layer206.

[0067] A gold (Au) electrode pad 208 is formed on the Ni film 207.Thereafter, a titanium (Ti) film is formed on part of the exposedsurface of the n-type GaN layer 202 and an aluminum (Al) film is formedon the Ti film to provide an n-side electrode 209.

[0068] The above-described fabrication process of the LED device 104will be described below in more detail. The sapphire substrate 201 isheated at a temperature of about 1050° C. in an atmosphere of H₂ usingan MOCVD apparatus for the purpose of subjecting the sapphire substrate201 to surface treatment. Thereafter, the temperature of the substrate201 is decreased to about 500° C. to form the GaN buffer layer (notshown). In this case, the thickness of the buffer layer is about 250 Å,for example. The substrate temperature is increased to about 1020° C.and the n-type GaN layer 202 having a thickness of about 4 μm is grownon the GaN buffer layer. The substrate temperature is decreased to about720° C. and the non-doped or Si-doped In_(0.2)Ga_(0.8)N active layer 203having a thickness of about 30 Å is grown on the n-type GaN layer 202.Thereafter, the p-type Al_(0.1)Ga_(0.9)N evaporation prevention layer204 having a thickness of about 100 Å is grown on the In_(0.2)Ga_(0.8)Nactive layer 203 at the same temperature (i.e., about 720° C.). Thesubstrate temperature is increased to about 900° C. and the p-typeAl_(0.1)Ga_(0.9)N upper evaporation prevention layer 205 having athickness of about 100 Å is grown on the p-type Al_(0.1)Ga_(0.9)Nevaporation prevention layer 204. The growth temperature is preferablyequal to, but actually is allowed to fall within a tolerable range ofabout plus or minus 50° C. from, the temperature at which theIn_(0.2)Ga_(0.8)N active layer 203 has been grown. Thereafter, thesubstrate temperature is increased to about 1000° C. and the p-type GaNcontact layer 206 having a thickness of about 3000 Å is grown on thep-type Al_(0.1)Ga_(0.9)N upper evaporation prevention layer 205.

[0069] A resist is applied on the p-type GaN contact layer 206 followedby patterning using a photolithography technique. Part of the resultantmulti-layer structure is dry-etched so that a portion of the n-type GaNlayer 202 is exposed. The n-type electrode 209 is formed on part of theexposed surface of the n-type GaN layer 202. After removal of theresist, the nickel (Ni) metal film 207 having a thickness of about 30 Åto about 100 Å is deposited on part of the p-type GaN contact layer 206by patterning using a photolithography technique. The gold (Au) padelectrode 208 having a thickness of about 4000 Å is provided in a waysimilar to that used when the metal film 207 is provided.

[0070]FIG. 3 is a graph showing a relationship between a driving currentand an emission wavelength when a direct driving current is applied tothe LED device 104 of Example 1. In FIG. 3, when DEVICE 1 is employed asthe LED device 104, as the driving current is increased, the emissionwavelength of the LED device 104 is shifted toward a shorter wavelength.That is, a blue shift phenomenon is observed. For example, when thedriving current is about 0.1 mA, the emission wavelength is about 508 nm(green). When the driving current is about 1 mA, the emission wavelengthis about 494 nm (blue-green). When the driving current is about 10 mA,the emission wavelength is about 481 nm (blue).

[0071] Thus, the emission wavelength is shifted by about 20 nm or moretoward a shorter wavelength in the case of a 10 mA drive as comparedwith the case of a 0.1 mA drive. The wavelength shift toward a shorterwavelength by about 20 nm or more causes visual observation of adifferent color. In general, for an LED device 104 having its emissionwavelength range in the visible spectrum, a change in emissionwavelength caused by a change in driving current needs to be suppressedto about 6 nm or less in order to maintain the same visual colorobserved.

[0072] The emission wavelength is herein defined as follows. Referringto FIG. 4, the emission wavelength spectrum of the LED device 104 has asingle peak and the half-value width of the emission wavelength is about30 nm, resulting in an emission characteristic having a satisfactorycolor purity. In the LED device 104 of Example 1, the peak wavelength ofthe emission wavelength spectrum represents the color of light emittedfrom the LED device 104. The peak wavelength of an LED device is hereindescribed as the emission wavelength of the LED device. For the sake ofsimplicity, an emission wavelength is used as a parameter of a lightcolor.

[0073]FIG. 5 is a graph showing the emission power characteristics ofthe LED device 104 of Example 1 when it is driven by a pulse current.The horizontal axis represents D×I (mA) where D is the duty D of thesquare wave pulse current R and I is the current value thereof. Thevertical axis represents the emission power P (μW) of the LED device104. The pulse herein means a periodic pulse as shown in FIG. 6.Referring to FIG. 6, the current value I represents a current value(peak value) of a pulse. The duty D is represented by T₂/T₁ where T₁ isthe period of a pulse and T₂ is the duration of a pulse. Therefore, theproduct D×I of the duty D and the current value I represents atime-average current injected to the LED device 104.

[0074] In Example 1, for the sake of simplicity of description of theprinciple of the present invention, it is assumed that the LED device104 is driven in a current region where the emission power issubstantially proportional to the current value. In this case, thetime-average emission power of the LED device 104 is proportional to theaverage current injected to the LED device resulting in a substantiallystraight line as shown in FIG. 5. That is, it is believed that theemission power P is substantially equal to or proportional to D×I.

[0075] In this case, it is preferable that when a viewer observes theLED device 104, he or she does not sense a flicker. To this end, theperiod T₁ is about 30 ms or less, more preferably about 10 ms or less,and the period T₂ is preferably as long as or longer than therecombination life span of a carrier in the active layer. In the casewhere the LED device 104 includes an InGaN active layer, the pulseduration is about 0.2 ns or more, and more preferably about 1 ns ormore.

[0076] The operation of the LED device of Example 1 will be describedbelow with reference to FIGS. 1 through 6. In this case, an LED device104 indicated by DEVICE 1 in FIGS. 3 and 5 has an emission wavelength ofabout 490 nm (blue-green) and an emission power of about 100 μW, forexample. The emission power is used as a parameter of emissionintensity. The LED on-off circuit 1 externally receives a color signal c(=c₄₉₀) indicating that the designated emission wavelength of the LEDdevice 104 is about 490 nm and an intensity signal p (=p₁₀₀) indicatingthat the designated emission power of the LED device 104 is about 100μW.

[0077] Initially, the color signal c is externally input to the colorsignal-current value converter 102. In the converter 102, the colorsignal c is converted to the current value signal i in accordance withdriving current versus emission wavelength characteristics shown in FIG.3 (DEVICE 1). When the color signal c (=c₄₉₀) is input to the converter102, the current value corresponding to an emission wavelength of about490 nm is about 2 mA in accordance with the driving current versusemission wavelength characteristics shown in FIG. 3 (DEVICE 1). Theconverter 102 generates a current value signal i (=i₂) indicating thatthe designated current value I of a pulse which should be output by thesquare wave pulse generator 103 is about 2 mA and sends the signal i tothe calculation processor 101 and the square wave pulse generator 103.

[0078] The calculation processor 101 receives the intensity signal p andthe above-described current value signal i, and calculates a duty D inaccordance with the D×I versus emission power characteristics shown inFIG. 5 (DEVICE 1) where I is a current value and D is a duty.

[0079] As is seen from FIG. 5 (DEVICE 1), the product D×I of the duty Dand the current value I is 1 mA with respect to the intensity signal p(=about 100 μW). As the current value indicated by the current valuesignal i is about 2 mA, the duty D is equal to about 0.5. Therefore, thecalculation processor 101 generates a duty signal d (=d_(0.5))indicating that the designated duty D of the pulse which the squarepulse generator 103 should output is about 0.5, and sends the dutysignal d to the square pulse generator 103.

[0080] The square pulse generator 103 generates and outputs a pulsecurrent R having a current value I which is equal to 2 mA and a duty Dwhich is equal to about 0.5, based on the current value signal i (=i₂)sent from the color signal-current value signal converter 102 and theduty signal d (=d_(0.5)) sent from the calculation processor 101. TheLED device 104 is driven by the pulse current R.

[0081] In this way, light having a designated emission wavelength andemission power can be emitted even when an LED device having an emissionwavelength which is dependent on a driving current value is employed.

[0082] The case where an LED device 104 indicated by DEVICE 2 in FIGS. 3and 5 outputs light having an emission power of about 100 μW and anemission wavelength of about 490 nm which are the same values asdescribed above, will be described. As is seen from the driving currentversus emission wavelength characteristics in FIG. 3, the emissionwavelength of DEVICE 2 is shifted by about 6 nm toward a longerwavelength at each current value as compared to that of DEVICE 1. As isseen from the D×I versus emission power characteristics show in FIG. 5,the product of the current value I and the duty D of DEVICE 2 is shiftedtoward the lower light output side as compared to that of DEVICE 1.

[0083] When the color signal-current value signal converter 102 receivesa color signal c (=c₄₉₀), the converter 102 generates a current valuesignal i (=i₅) designating a current of about 5 mA corresponding to anemission wavelength of about 490 nm in accordance with the drivingcurrent versus emission wavelength characteristics in FIG. 3 (DEVICE 2),and sends the current value signal i to the calculation processor 101and the pulse generator 103.

[0084] The calculation processor 101 receives the current signal p(=p₁₀₀) and the current value signal i (=i₅). As is seen from the D×Iversus emission power characteristics show in FIG. 5, the product D×I ofthe duty D and the current value I is about 3 with respect to theintensity signal p (=about 100 μW). As a current value indicated by thecurrent value signal i is about 5 mA, the calculation processor 101calculates that the duty D is equal to about 0.6.

[0085] The calculation processor 101 generates a duty signal d(=d_(0.6)) indicating that the designated duty D of the pulse which thepulse generator 103 should output is equal to about 0.6, and sends theduty signal d to the pulse generator 103.

[0086] The pulse generator 103 generates and outputs a pulse current Rhaving a current value I of about 5 mA and a duty D of about 0.6, basedon the current value signal i (=i₅) sent from the color signal-currentvalue signal converter 102 and the duty signal d (=d_(0.6)) Sent fromthe calculation processor 101. The LED device 104 is driven by the pulsecurrent R. In this way, light having a designated emission wavelengthand emission power can be emitted by driving the LED device 104 with thepulse current R.

[0087] As described above, with the driving method of the presentinvention, the LED device having an emission wavelength which is changedwith a driving current value can be driven so that the LED deviceoutputs light having a constant wavelength and power. When the LEDdevices having emission wavelengths which are changed with drivingcurrent values are driven at a predetermined driving current value, theemission wavelengths vary among the LED devices due to variations incharacteristics of the LED devices, resulting in variations in colortones. Such a problem is solved by the present invention.

[0088] The relationship of the D×I versus the emission power shown inFIG. 5 may be stored as a table or function in the calculation processor101. The relationship between the current value versus the emissionwavelength shown in FIG. 3 corresponding to a target LED device may bestored in the color signal-current value signal converter 102.

[0089] Although the above-described pulse wave is a square wave inExample 1, the pulse waveform generated by the pulse generator 103 isnot limited to the square wave, but may be any form including atriangular wave. The pulse wave may be generated in the following way:(1) the duty is modified by changing the pulse duration while the periodis maintained constant; (2) the duty is modified by changing the periodwhile the pulse duration is maintained constant; or (3) the duty ismodified by changing the number of pulses in a predetermined period oftime.

[0090] The pulses may be concentrated in the first half of apredetermined period (a in FIG. 18), the pulses may be concentrated onthe letter half of a predetermined period (b in FIG. 18), or the pulsesare distributed in the entire predetermined period (a in FIG. 18). Thatis, as long as the average power of the driving current is constant, theshapes, the widths or the number of individual pulses may be arbitrarilyselected using a simple method. As described above, there are thevarious methods for generating pulses, but the present invention is notlimited to those methods. In the present invention, other pulsegeneration methods may be used.

EXAMPLE 2

[0091] In Example 2, the LED apparatus (including DEVICE 1) of Example 1is employed. An attempt is made, where the emission intensity of the LEDdevice 104 is temporally changed from about 50 μW to about 100 μW toabout 200 μW while the emission wavelength of the LED device 104 ismaintained constant. In such an attempt, the color signal c isconstantly equal to c₄₉₀, and the intensity signal p is changed from p₅₀to p₁₀₀ to p₂₀₀.

[0092] Since the color signal c is constantly equal to c₄₉₀, the colorsignal-current value signal converter 102 outputs the same current valuesignal i as that in Example 1, i.e., i=i₂.

[0093] The duty signal d output from the calculation processor 101 iscalculated in a procedure similar to that in Example 1. As the intensitysignal p is changed from p₅₀ to p₁₀₀ to p₂₀₀, the duty signals d(=d_(0.25), d_(0.5), and d₁), which cause the duty D to be about 0.25,about 0.5, and about 1, respectively, are sequentially sent to the pulsegenerator 103.

[0094] Thus, in such a procedure similar to that in Example 1, the LEDdevice 104 outputs light having a intensity which is changed temporallyand a constant wavelength, the intensity and the wavelength beingexternally designated.

[0095] In Example 2, although the LED device having an emissionwavelength (color tone) which varies depending on a driving currentvalue is employed, the emission intensity can be solely changed whilethe emission wavelength (color tone) is maintained constant.

[0096] Such a driving method may be applied to a display apparatus whichexhibits a color tone by simultaneously employing a plurality of LEDdevices having different light colors. In this case, color shift can besuppressed. Alternatively, the driving method may be applied to adisplay apparatus in which a fluorescent is excited by the LED device sothat the fluorescent emits light. In this case, a reduction in emissionefficiency due to excitation wavelength shift, or color tone shift canbe prevented. Even when an LED device having a characteristic differentfrom the LED device in Example 2 is employed (there are variations in acharacteristic), the emission intensity can be solely changed while apredetermined light color is maintained in a way and for the reasonssimilar to those described in Example 1.

EXAMPLE 3

[0097] In Example 3, a simplified variant of the LED on-off circuit ofExample 1 is provided. Referring to FIG. 7, the LED on-off circuit ofExample 3 is comprised of only a square wave pulse generator 103 a. Itis only the intensity signal p that is externally input to the LEDon-off circuit. The square wave pulse generator 103 a only modifies theduty of a current pulse which is to be supplied to the LED device 104based on the value of the intensity signal p while maintaining the peakvalue of a current at a predetermined constant value. If a change inwavelength (blue shift) of the LED device 104 is controlled so that therange of change is about 6 nm or less, variation in the peak value of acurrent does not substantially cause problems. In this case, the peakvalue of a current is regarded as being constant. When the peak value ofa current is set to the maximum of the range in which the reliability ofthe LED device 104 can be secured, the dynamic range of the emissionintensity of the LED device 104 can be most preferably maximized.Specifically, the peak value of a pulse current is preferably set toabout 10 mA or more and about 300 mA or less.

[0098] Even when the LED on-off circuit having such a simpleconfiguration is employed, the LED device 104 can output light having anemission power in the range from about 0.1 mW to about 5.0 mW withrespect to a pulse duty in the range from about 0.2% to about 100%. Inthis case, the emission wavelength of the LED device 104 is decreased byabout 4 nm, i.e., from about 472 nm to about 468 nm. Such a change(i.e., about 4 nm) is less than about 6 nm at and above which a changein emission wavelength is visually recognized as a change in color.

[0099] Even when the calculation processor 101 of Example 1 using thecolor signal c and the color signal-current value signal converter 102performing color control are not employed, a change in the light colorof the LED device 104 due to the blue shift thereof can be significantlysuppressed by controlling the emission power of the LED device 104 inthe following way. A pulse having a constant peak value is input to theLED device 104 so that a change in the emission wavelength of the LEDdevice 104 is less than about 6 nm, and a pulse duty is changed.Thereby, a change in the light color of the LED device 104 can bereduced to an extent in which a change in color is not visuallyrecognizable.

[0100] When the peak value of a pulse current is set to the maximum ofthe range in which the reliability of the LED device 104 can be secured,the dynamic range of the emission intensity of the LED device 104 can bemaximized.

EXAMPLE 4

[0101] In Example 4, the LED apparatus (including DEVICE 1) of Example 1is employed. An attempt is made, where the emission wavelength of theLED device 104 is temporally changed from about 475 nm (blue) to about485 nm (blue to blue-green) to about 494 nm (blue-green to green) whilethe emission intensity of the LED device 104 is maintained constant atabout 60 μW. In such an attempt, the emission intensity signal p isconstantly equal to p₆₀, and the color signal c is changed from c₄₇₅ toc₄₈₅ to c₄₉₄.

[0102] The color signal-current value signal converter 102 convertscolor signals c (=c₄₇₅, c₄₈₅, and c₄₉₄) to current value signals i(=i₃₀, i₄, and i₁) designating current values of about 30 mA, about 4mA, and about 1 mA in accordance with FIG. 3 similar to Example 1.

[0103] The duty signal d output from the calculation processor 101 iscalculated in a procedure similar to that in Example 1. As the currentvalue signal p is changed is from i₃₀ to i₄ to i₁, the duty signals d(=_(0.02), d_(0.15), and d_(0.6)), which cause the duty D of the squarewave pulse current R to be about 0.02, about 0.15, and about 0.6,respectively, are sequentially sent to the pulse generator 103.

[0104] Thus, in such a procedure similar to that in Example 1, the LEDdevice 104 outputs light having a wavelength which is changed temporallyand a constant intensity, the intensity and the wavelength beingexternally designated.

[0105] With the driving method of the present invention, the light colorof an LED device can be changed without changing the emission intensitythereof. Thus, when the light color of an LED device is changed usingthe method of Example 4, a current value is preferably changed by afactor of about 10 or more in order to change a color tone, and morepreferably by a factor of about 20 or more.

[0106] The driving method of Example 4 may be applied to a displayapparatus. Since the emission wavelength can be changed using a singleLED device without changing the emission intensity. Thereby, a displayapparatus having a simple configuration and a significant visual effectcan be achieved using the driving method of Example 4. The light colorof an LED device can be continuously changed with ease by continuouslychanging a color signal which is to be input to an LED on-off circuit.

[0107] Further, in a combination of Examples 2 and 3, light having anarbitrary set of different wavelength and intensity values, e.g., about490 nm and about 200 μW, and about 475 nm and about 60 μW, can beemitted by DEVICE 1, for example.

[0108] In Examples 1 through 4, for the sake of simplicity, theoperation of the LED device, in which the current value is proportionalto the emission power, is described. The present invention is notlimited to such an LED device there are a number of actual LED deviceswhich have such a proportional relationship. FIG. 8 shows current versusemission power (instantaneous value) characteristics of such an LEDdevice. In FIG. 8, the current value is not proportional to the emissionpower, so that the graph is a curve. In this case, the D×I versusemission power characteristics as shown in FIG. 5 cannot be drawn with asingle line, but a different line must be used for each current value I.

[0109] An LED device having the current versus emission powercharacteristics shown in FIG. 8 is employed. Using the method of Example1, the LED device is caused to emit light having a wavelength of about490 nm and a power of about 100 μW which are the same target values asthat in Example 1.

[0110] The color signal c (=c₄₉₀) is transferred to the colorsignal-current value signal converter 102 which in turn outputs acurrent value signal i (=i₂) designating a current value of about 2 mA.As is seen from FIG. 8, when the current value is about 2 mA, an LEDdevice outputs light having a power of about 300 μW (instantaneousvalue). Therefore, the calculation processor 101 calculates a duty Dwhich is equal to about {fraction (1/3)} (=100 μW/300 μW), and outputs aduty signal d corresponding to the duty D. In the foregoing, it isconsidered that the duty D is proportional to the emission power P.However, when heat release is insufficient, if the product D×I of theduty D and the current value I is increased, the emission efficiency isdecreased due to generated heat.

[0111] In this case, a proportional relationship is not necessarilyestablished between the duty D and the emission power P. Nevertheless,the emission power P is represented by a function having the duty D andthe current value I as parameters (or D is a function having P and I asparameters). The function is obtained by determining characteristics ofan LED device in advance. The calculation processor 101 calculates thefunction based on the LED device 104 and outputs a duty signal dcorresponding to the resultant duty D. Alternatively, a relationshipbetween the emission power P, the duty D, and the current value I may bestored as a table. In this way, the driving method of the presentinvention can drive an LED device having current versus emissionintensity characteristics are not linear.

[0112] In Examples 1 through 4, the color of light is represented usingthe peak value of a light spectrum as a parameter as described inExample 1. This is done for the sake of simplicity. This is notessential, however, to the present invention as described in Examples 1through 4. For example, when a CIE standard chromaticity diagram isemployed to represent the light color, the essence of the presentinvention is not impaired.

[0113] Further, in Examples 1 through 4, for the sake of simplicity, theemission intensity of an LED device is represented by the emission power(W), but another parameter may be employed. When a parameter in whichvisibility is taken into account, such as luminance (cd/m²), luminousintensity (cd), or luminous flux (lm), is employed, if an LED device isdriven in such a manner that the emission wavelength is changed whilethe emission intensity is maintained constant, apparent brightness isadvantageously not changed due to a change in the emission wavelength.In this case, however, the relationship between the product D×I of theduty D and the current value I and the emission power cannot berepresented by a single line on a graph. Nevertheless, the drivingmethod of the present invention can drive an LED device having currentversus emission intensity characteristics are not linear. To this end,the relationship among the emission power P, the duty D, and the currentvalue I is stored in the calculation processor 101 where the unit of theemission power P is different from that in Examples 1 through 4. Theessence of the present invention described in Examples 1 through 4 doesnot depend on whether the current versus emission intensitycharacteristics of an LED device are linear or nonlinear.

[0114] Further, in Examples 1 through 4, the specific LED devices(DEVICE 1 and DEVICE 2) are employed, but the present invention can beapplied to any LED device having a wavelength which is decreased with anincrease in current (a blue shift occurs in which the difference inwavelength between the minimum driving current and the maximum drivingcurrent set in a driving circuit is about 6 nm or more).

[0115] For example, in an LED device having almost the sameconfiguration as that of the LED device shown in FIG. 2, when thecomposition of an InGaN active layer thereof is modified in such amanner that the current value is changed from about 0.1 mA to about 100mA, the emission wavelength can be changed from about 590 nm to about530 nm. The present invention can also be applied to an LED device inwhich an InGaN active layer or InGaAlN active layer is provided on aconductive (preferably n-type) GaN substrate. When these LED devices areemployed in the configuration of Examples 1 through 4, an LED apparatuswhich emits light having a predetermined color from orange to yellow toyellow-green to green can be achieved.

EXAMPLE 5

[0116]FIG. 9 is a diagram illustrating an LED bullet according toExample 5 of the present invention. The LED bullet of Example 5 includesan LED device 801 having a light color which is changed with a drivingcurrent value and a fluorescent 802 provided in a light emittingdirection of the LED device 801. Light output from the LED device 801and light output from the fluorescent 802 are mixed, and the mixed lightis output from the LED bullet. The LED device 801 is the same as that inExample 1.

[0117] The operation of the LED bullet of Example 5 will be describedbelow. FIG. 10 is a graph showing excitation and emission spectra whenthe fluorescent 802 is excited at about 460 nm. The peak of theexcitation spectrum of the fluorescent 802 used in Example 5 is about460 mm. It has been found that when the fluorescent 802 is irradiatedwith light having a wavelength of about 460 nm, the fluorescent 802emits light having the emission spectrum shown in FIG. 10 with the mostefficiency. In Example 5, the peak wavelength of the emission spectrumis about 630 nm, which indicates red light. An example of a fluorescenthaving such a characteristic is a YAG fluorescent.

[0118] It is assumed that the fluorescent 802 is excited by the LEDdevice 801 having the current versus emission wavelength characteristicsshown in FIG. 3 (DEVICE 1) As a driving current supplied to DEVICE 1 ischanged from about 0.1 mA to about 2 mA to about 30 mA, the emissionwavelength of DEVICE 1 is changed from about 508 nm (green) to about 490nm (blue-green) to about 474 nm (blue), respectively.

[0119]FIG. 11 is a CIE standard chromaticity diagram. In FIG. 11, a lineindicated by DEVICE 1 represents the light color of DEVICE 1 when thedriving current value is changed from about 0.1 mA to about 2 mA toabout 30 mA. A point indicated by FLUORESCENT 103 represents the lightcolor of the fluorescent 802 which has the emission spectrum shown inFIG. 10. As is seen from the excitation spectrum of the fluorescentshown in FIG. 10, when the emission wavelength of DEVICE 1 is about 508nm, the driving current supplied to DEVICE 1 is small, so that theemission power is small. In this case, the emission wavelength islocated at an end of the excitation spectrum, so that the fluorescent802 is substantially not excited. In contrast, when the emissionwavelength of DEVICE 1 is about 474 nm, the driving current is large, sothat the emission power is large. In this case, the emission wavelengthis located in the vicinity of the peak of the excitation spectrum, sothat DEVICE 1 is significantly excited to emit light. Thus, the shorterthe emission wavelength of DEVICE 1, the more significantly thefluorescent is excited.

[0120] For example, in the LED bullet of Example 5, when the LED device801 is driven at about 0.1 mA, the LED device 801 emits light having awavelength of about 508 nm, but the fluorescent is substantially notexcited. In this case, the LED device 801 substantially solely emitslight (green) in the LED bullet.

[0121] When the LED device 801 is driven at about 30 MA, light (i.e.,blue, about 474 nm) emitted from the LED device 801 and light (i.e.,red, about 630 nm) emitted from the fluorescent 802 excited by lightemitted from the LED device 801 are mixed, so that the mixed light isred violet.

[0122] When the LED device 801 is driven at about 2 mA an emissionwavelength of about 490 nm (blue-green) is obtained, even though theemission power of the LED device 801 is not large, and the emission ofthe fluorescent 802 is not very large since the emission wavelength isin the vicinity of an edge of the emission spectrum. Light emitted fromthe fluorescent 802 and light (i.e., blue-green, about 490 nm) emittedfrom the LED device 801 are mixed, so that the mixed light is white.

[0123] Thus, the light color of the LED bullet when the current value ischanged from about 0.1 mA to about 2 mA to about 30 mA is shown with aline indicated by EXAMPLE LED in the CIE standard chromaticity diagramof FIG. 11. Thus, when the current value is changed from about 0.1 mA toabout 2 mA to about 30 mA, the light color of the LED bullet of Example5 is changed from green to white to red violet, respectively.

[0124] As described above, in Example 5, the combination of the LEDdevice having a light color which is changed with a current and thefluorescent allows the LED bullet to achieve the change in the lightcolor due to a change in current that is not obtained using only an LEDdevice.

[0125] In Example 5, the specific fluorescent shown in FIG. 10 isemployed. There are various types of fluorescents. Appropriatecombinations of LED devices and fluorescents can lead to variations inlight color due to a change in current.

EXAMPLE 6

[0126] Similar to FIG. 9, the LED bullet of Example 6 includes an LEDdevice 801 having a light color is changed with a driving current value,and a fluorescent 802 provided in a light emitting direction of the LEDdevice 801. Light output from the LED device 801 and light output fromthe fluorescent 802 are mixed, and the mixed light is output from theLED bullet. The operation of the LED device 801 of Example 6 will bedescribed below.

[0127]FIG. 12 is a graph showing the current versus emission wavelengthcharacteristics of the LED device of Example 6. In FIG. 12, DEVICE 3indicates the LED device in which the emission wavelength is changedfrom about 450 nm (blue) to about 430 nm (blue) to about 415 nm(blue-violet) as the current value is respectively changed from about0.1 mA to about 2 mA to about 30 mA. DEVICE 3 can be obtained bychanging the composition of the InGaN layer of the LED device shown inExample 1 and FIG. 2. FIG. 13 is a CIE1960UCS chromaticity diagram. Achromaticity coordinate (U. V) in the CIE1960USC chromaticity diagramhas a relationship with the corresponding chromaticity coordinate (x, y)in the CIE standard chromaticity diagram, the relationship being:

u=4x/(−2x+12y+3), and

v=6y/(−2x+12y+3).

[0128] The UCS chromaticity diagram has a characteristic that thedistance between two coordinate points having different colorscorresponds to a color difference visually recognized. In FIG. 13, aline indicated by LED device shows a change in the light color of theLED device of Example 6 when a driving current supplied to the LEDdevice is changed from about 0.1 mA to about 30 mA. In this case, (u, v)is respectively changed from about (0.25, 0.03) to about (0.19, 0.06).Therefore, the distance between the coordinate points (color difference)is about 0.07.

[0129]FIG. 14 is a graph showing the excitation spectrum and emissionspectrum of a fluorescent of Example 6. In FIG. 14, the peak wavelengthof the excitation spectrum is about 430 nm. When the fluorescent asirradiated with light having such a wavelength, the fluorescent emitslight having the emission spectrum with the most efficiency. In FIG. 14,the peak wavelength of the emission spectrum is about 650 nm having acolor which is red. Such a fluorescent can be made of 6MgO.As₂O₅: Mn⁴⁺.In FIG. 13, a coordinate point (u, v) (w(0.55, 0.31)) indicated byFLUORESCENT represents the light color of the fluorescent of Example 6.

[0130] According to the excitation spectrum of the fluorescent shown inFIG. 14, when the emission wavelength of DEVICE 3 is about 450 nm (0.1mA), the emission power of the LED device is small, so that thefluorescent is substantially not excited. However, as the emissionwavelength of DEVICE 3 becomes shorter (e.g., 415 nm (30 mA)), theemission power of the LED device becomes larger while light emitted fromthe LED approaches the peak of the excitation spectrum of thefluorescent.

[0131] Therefore, the fluorescent is significantly excited to emitlight. The LED bullet emits blue light which is dominated by lightemitted from the LED device when the LED device is driven at about 0.1mA.

[0132] When the LED device is driven at about 30 mA, light (i.e., red,about 660 nm) emitted from the fluorescent and light (i.e., blue-violet,about 415 nm) emitted from the LED device are mixed. Since thevisibility of the light color of the LED device is low, the light (i.e.,red light, about 660 nm) of the fluorescent is dominant. Thereby, red isvisually recognized.

[0133] Further, when DEVICE 3 is driven at about 2 mA, the emissionwavelength of DEVICE 3 is about 430 nm, i.e., blue. In this case,whereas the emission power of the LED device is very large, the emissionof the fluorescent is not very large. The light is mixed to be violet.

[0134] The light color of the LED bullet when the driving current valueis changed from about 0.1 mA to about 2 mA to about 30 mA is shown witha line indicated by LED BULLET in the UCS chromaticity diagram of FIG.13. The color tone (u, v) of the LED bullet is respectively changed fromabout (0.25, 0.1) to about (0.53, 0.28). The distance (color difference)between the coordinate points is about 0.33.

[0135] Thus, when the current value is modulated, the color tone of theLED bullet of Example 6 is changed more significantly than the colortone of the LED device used in Example 6. This is because thefluorescent used in Example 6 is one having an excitation efficiencywhich is significantly changed (by a factor of about 2 or more, morepreferably about 5 or more) as the emission wavelength of the LED deviceis changed.

[0136] As described above, an LED device having a light color which ischanged with a current value is combined with a fluorescent having anexcitation efficiency which is significantly changed as the emissionwavelength of the LED device is changed. The resultant LED bullet has amore significant change in a light color with a current value than achange in a light color of the LED device.

EXAMPLE 7

[0137] Similar to Example 1, a light emitting apparatus according toExample 7 of the present invention includes: a light emitting sectionfor emitting light in which a color of the light is blue shifted with avalue of a driving current; and a driving section for driving the lightemitting section so that the light emitting section emits light having adesired color and a desired intensity, by generating the driving currentbased on a signal designating the desired color and a signal designatingthe desired intensity and by applying the driving current to the lightemitting section. In description of Example 7, the light emittingsection serves as an LED bullet, and the driving section serves as anLED on-off circuit.

[0138] An LED apparatus according to Example 7 will be described below.In the LED apparatus, the LED bullet of Example 5 is driven using an LEDon-off circuit similar to that of Example 1. The basic structure of theLED on-off circuit of Example 7 is the same as that shown in FIG. 1, butis newly shown in FIG. 17. The LED apparatus of Example 7 differs fromthat of Example 1 in that an LED device 164 is an LED bullet in which afluorescent is provided in a light emitting direction of an LED deviceas described in Example 5. Further, in FIG. 17, an LED on-off circuit 16receives a luminance signal k designating the emission intensity of theLED bullet 164 and a chromaticity signal u designating the light colorof the LED bullet 164, and outputs a pulse current R having a currentvalue of I and a duty of D. The LED bullet 164 is driven in accordancewith the pulse current R. The LED on-off circuit 16 includes: achromaticity signal-current value signal converter 162 for convertingthe chromaticity signal u to a current value signal i designating acurrent value I; a calculation processor 161 for calculating andoutputting a duty signal d designating a duty D based on the luminancesignal k and the current value signal i; and a square wave pulsegenerator 163 for outputting a square wave pulse current R as an LEDdriving current based on the current value signal i and the duty signald. The square wave pulse current R has a current value of I and a dutyof D.

[0139]FIG. 16 is a graph showing the driving current versus chromaticity(u value) characteristics of the LED bullet 164 of Example 7. The uvalue is a value on the u axis on the USC chromaticity diagram which isdetermined based on the light color corresponding to the peak wavelengthof the emission wavelength spectrum of the LED bullet 164. For the sakeof simplicity, the u value is used as a parameter of the light color ofthe LED bullet 164.

[0140] As is seen from the excitation spectrum of a fluorescent shown inFIG. 10, when the excitation wavelength is about 508 nm (0.1 mA), thefluorescent is substantailly not excited. As the excitation wavelengthbecomes shorter toward about 474 nm (30 mA), the fluorescent is moresignificantly excited. Therefore, when the LED bullet 164 is driven atabout 0.1 mA, light emitted from the LED device is substantiallydominant, so that light emitted from the LED bullet becomes blue.

[0141] When the LED device is driven at about 30 mA, light (i.e., red,about 660 nm) emitted from the fluorescent and light (i.e., blue-violet,about 415 nm) emitted from the LED device are mixed. In this case, sincethe visibility of the light color of the LED device is low, the light(i.e., red light, about 660 nm) of the fluorescent is dominant. Thereby,red is visually recognized.

[0142] When the LED device is driven at about 2 mA, the emission of thefluorescent is not very significant and light emitted from thefluorescent is mixed with light (i.e., blue, about 430 nm) emitted fromthe LED device, so that light emitted from the LED bullet becomesviolet.

[0143] The light color of the LED bullet when the current value ischanged from about 0.1 mA to about 2 mA to about 30 mA is shown with aline indicated by LED BULLET in the UCS chromaticity diagram of FIG. 13.

[0144]FIG. 16 is a graph showing the relationship between the currentvalue and the u value. As is seen from FIG. 16, when the driving currentis about 0.1 mA, the u value is equal to about u_(0.25), when thedriving current is about 1 mA, the u value is equal to about u_(0.37),and when the driving current is about 1.0 mA, the u value is equal toabout u_(0.475).

[0145]FIG. 15 is a graph showing the luminance characteristics of theLED bullet of Example 7. The horizontal axis represents the currentvalue I of the square wave pulse current R, and the vertical axisrepresents the luminance (mcd) of the LED bullet. A characteristic curveis shown for each duty D. That is, FIG. 15 is a graph showing therelationship between the luminance L and current value I of the LEDbullet of Example 7.

[0146] The pulse period T₂ is not particularly specified, however, it ispreferable that a viewer does not sense a flicker. To this end, theperiod T₂ is about 30 ms or less, more preferably about 10 ms or less.

[0147] The operation of the LED apparatus and the LED on-off circuit ofExample 7 will be described in detail with reference to FIGS. 14, 15,16, and 17. The LED device having the characteristics shown in FIGS. 15and 16 is operated so that the U value is equal to u_(0.37) and theemission intensity is equal to about 100 mcd, for example. Thus,luminance is used as a parameter of the emission intensity. Thechromaticity signal u (=u_(0.37)) and the luminance signal k (=k₁₀₀)designating the luminance (about 100 mcd) of the LED device areexternally input to the LED on-off circuit 17.

[0148] Initially, the chromaticity signal u is externally input to thechromaticity signal-current value signal converter 162. In this case,the chromaticity signal u is converted to the current value signal i inaccordance with the driving current versus chromaticity characteristicsshown in FIG. 16. When the chromaticity signal u (=u_(0.37)) is input tothe converter 162, the converter 162 outputs the current value signal i(=i₁) indicating that the designated current value of a pulse to beoutput by the pulse generator 163 is about 1 mA. This is because thecurrent value versus chromaticity characteristics shown in FIG. 16indicates a current value of about 1 mA with respect to a chromaticityof about 0.37. The current value signal i (=i₁) is sent to thecalculation processor 161 and the pulse generator 163.

[0149] The calculation processor 161 receives the luminance signal k andthe current value signal i. In the calculation processor 161, a duty Dis calculated in accordance with the luminance characteristics of theLED bullet shown in FIG. 15. In this case, the luminance signal X isk₁₀₀, and the current value signal i is i₁. As can be seen from FIG. 15,the duty D corresponding to the luminance k (=about 100 mcd) and i₁(=about 1 mA) is about 0.5. The calculation processor 161 calculatesthat the duty D is about 0.5, and outputs to the pulse generator 163 aduty signal d (=d_(0.5)) indicating that the designated duty D to beoutput by the pulse generator 163 is about 0.5.

[0150] The pulse generator 163 generates and outputs a pulse current Rhaving a current value of about 1 mA and a duty of about 0.5, inaccordance with the current value signal i (=i₁) sent from thechromaticity signal-current value signal converter 162 and the dutysignal d (=d_(0.5)) sent from the calculation processor 161, and the LEDbullet 164 is driven by the pulse current R to emit light.

[0151] In this way, the LED device dependent on the u value of thechromaticity characteristics with respect to the driving current canemit light having the designate luminance and u value on the USCchromaticity diagram.

[0152] Further, an LED device (having variations in characteristics inFIGS. 15 and 16) different from the above-described LED devices can bemade to emit light having a predetermined luminance and chromaticity asshown in Example 7.

EXAMPLE 8

[0153] In Example 8, the LED apparatus of Example 7 is employed. Anattempt is made, where the luminance of the LED bullet 164 to temporallychanged from about 10 mcd to about 40 mod to about 100 mcd while the Uvalue of the chromaticity characteristic of the LED device 164 ismaintained constant at about 0.37 as in Example 7.

[0154] In this case, the chromaticity signal u is maintained constant atu_(0.37), and the luminance signal k is changed from k₁₀ to k₄₀ to k₁₀₀.

[0155] The chromaticity signal-current value signal converter 162outputs the same current value signal i (=d₁) as that described inExample 6.

[0156] The duty signal d output from the calculation processor 161 iscalculated in a procedure similar to that described in Example 6.Therefore, as the luminance signal k is changed from k₁₀ to k₄₀ to k₁₀₀,the duty signal d which is output to the pulse generator 163 isrespectively changed from d_(0.02), d_(0.1), and d_(0.42) designatingduties D Which are equal to about 0.02, about 0.1, and about 0.42,respectively.

[0157] As a result, the LED bullet 164 is driven in accordance with thetime-varying luminance signals k and the constant chromaticity signal u(note that, in this case, light having a luminance of about 180 mcd ormore cannot be obtained).

[0158] In Example 8, although the LED device having an emissionwavelength (color tone) which is changed with a driving current value isemployed, the emission intensity can be solely changed while theemission wavelength (color tone) is maintained constant. Even in LEDdevices differing from the above-described LED devices incharacteristics, the emission intensity may be solely changed while thelight color is maintained to be a predetermined color, using a similarmethod and for similar reasons as those described in Example 1.

[0159] When the above-described driving method is applied to a displayapparatus in which LED devices having different light colors aresimultaneously controlled so as to provide color tones, color shift canbe suppressed.

EXAMPLE 9

[0160] In Example 9, the LED apparatus of Example 7 is employed. Anattempt is made, where the u value of the chromaticity characteristic ofthe LED bullet 164 is temporally changed from about 0.25 to about 0.37to about 0.51 while the luminance of the LED device 164 is maintainedconstant at about 40 mcd.

[0161] In this case, the luminance signal k is maintained constant atk₄₀, and the chromaticity signal u is changed from u_(0.25) to u_(0.37)to u_(0.51),

[0162] The chromaticity signals u (=u_(0.25), u_(0.37), and u_(0.51))are converted by the chromaticity signal-current value signal converter162 to the current value signals i (=i_(0.01), i₁, and i₂₀) designatingcurrent values of about 0.01 mA, about 1 mA, and about 20 mA, inaccordance with FIG. 16 as described In Example 7.

[0163] The duty signal d output from the calculation processor 161 iscalculated in a procedure similar to that described in Example 7.Therefore, as the current value signal i is changed from i_(0.01) to i₁to i₂₀, the duty signal d which is output to the pulse generator 163 ischanged from d₁, d_(0.22), and d_(0.1) designating duties D which areequal to about 1, about 0.22, and about 0.1, respectively.

[0164] As a result, the LED bullet 164 is driven in accordance with thetime-varying chromaticity signal u and the constant luminance signals k.

[0165] With the driving method of the present invention, the light colorof an LED device can be changed without changing the luminance thereof.Thus, when the light color of an LED device is changed using the methodof Example 9, a current value is preferably changed by a factor of about10 or more in order to change a color tone, more preferably by a factorof about 20 or more.

[0166] When the above-described driving method is applied to a displayapparatus, the u value of the chromaticity characteristic of a singleLED device can be changed without changing the luminance thereof. Theeyes of a human being can recognize that light having any wavelength hasthe same brightness, and therefore, significant visual effects can beachieved using a display apparatus having a simple configuration.Further, the light color of the LED device can be continuously changedwith ease by continuously changing color signals input to the LED on-offcircuit.

[0167] In Example 9, an LED bullet in which a fluorescent is excited byan LED device is employed. The light color of the LED bullet can bechanged from blue to red without changing the luminance by changing thevalue of a current supplied to the LED bullet using the LED on-offcircuit and the driving method thereof described in Example 1. Such arange of variation in light color in larger than that obtained by aconventional LED apparatus.

[0168] In Examples 1 through 9, the driving current supplied to the LEDdevices is a periodic pulse current. The fundamental period of thedriving current is preferably about 30 ms or less, more preferably 10 nsor less, so as to prevent a viewer from sensing a flicker. Further, thepulse duration is about 0.2 ns or more, more preferably 1 ns or more, asdescribed above so that the carrier density can be reliably controlled.

[0169] In Examples 1 through 9, the duty is changed while the currentvalue is maintained constant, or the current value is changed while theduty is maintained constant. Nevertheless, the current value or the dutymay be changed If the color category of the resultant light color is notsubstantially changed (plus or minus 3 nm or less of a desiredwavelength).

[0170] In Examples 5 through 9, a fluorescent may be appropriatelyselected to obtain the desired excitation spectrum and emissionspectrum.

[0171] Examples of a fluorescent for emitting red light include ZnS:Cu,LiAlO₂:Fe³⁺, Al₂O₃:Cr, Y₂O₃:Eu³⁺, Y(P,V)O₄:Eu³⁺, Y₂O₃:Eu, and acombination of Y₂O₃:Eu and Y₂O₃S:Eu.

[0172] Examples of a fluorescent for emitting orange light includeZnSiCu, Mn, (Zn, Cd)S:Ag, ZnS:Mn, and (Sr,Mg,Ba)₃(PO₄)₂.

[0173] Examples of a fluorescent for emitting green light includeZnS:Cu, Al, LaPO₄:Ce³⁺Tb³⁺, Sr(S,Se):Sm, Ce, ZnSiO₄:Mn²⁺, βZnS:Cu,ZnS:Cu, Fe(Co), ZnS:PbZnS:Cu, and a combination of ZnS:Cu, Al, andY₂Al₅O₁₂:Tb.

[0174] Examples of a fluorescent for emitting blue light include CaS:Bi,(Sr,Ca)₁₀(PO₄)₆Cl₂: Eu²⁺, SrS:Sm, Ce, Sr₂P₂O₇:Eu²⁺, βZnS:Ag,(Ba,Ca,Mg)₁₀(PO₄)₆Cl:Eu²⁺, and 3Sr₃(PO₄)₂.CaCl₂:E²⁺.

[0175] Examples of a fluorescent for emitting white light includeZnO:Zn, ZnS:AsZnS:Au, Ag, Al, Ca₂P₂O₇:Dy, Ca₃(PO₄)₂.CaF₂:Sb,3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb³⁺, 3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb³⁺, Mn²⁺, MgWO₄,3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb²⁺, and Mn².

EXAMPLE 10

[0176] In Example 10, a display apparatus is obtained using theabove-described LED driving method or LED apparatus. FIG. 19 is aschematic diagram illustrating a display employing the above-describedLED on-off circuit or LED on-off method. In FIG. 19, the LED on-offcircuit of the above-described examples is incorporated Into a display171. Alternatively, the LED on-off circuit maybe separated from thedisplay 171 (i.e., the LED on-off circuit do not need to be incorporatedinto the display 171).

[0177] In FIG. 19, LED devices 172 (or LED bullets) are provided along Xand Y directions on a plane so as to form a matrix. When the LED devices172 are driven by a conventional LED driving circuit variations in theis characteristics of each LED device 172 are significant. Therefore,variations in the color and brightness of light result in adeterioration in the picture quality of the display. In contrast, whenthe display apparatus shown in FIG. 19 includes the LED apparatusdescribed in Examples 1 through 9 or employs the LED driving methoddescribed therein, if the LED devices 172 are driven by the respectiveLED on-off circuit connected thereto, the LED devices having variationsin the characteristics are all allowed to emit the desired light colorand brightness. Thereby, the picture quality of an image provided by thedisplay apparatus can be improved.

[0178] According to the present invention, an LED apparatus having alight color which is not changed even if the emission intensity ischanged, can be achieved where the light color of an LED device includedin the LED apparatus is changed with a change in a driving currentvalue.

[0179] A plurality of LED devices which have light colors which arechanged with driving current values and which have variations in thecharacteristics can be driven so that the same emission wavelength andthe same emission intensity are obtained. Conventionally, when the LEDdevices having light colors which are changed with changes in drivingcurrent values are employed, the emission wavelengths (i.e., colortones) of the LED devices are different from one another due tovariations in characteristics thereof. Such a problem is solved by thepresent invention.

[0180] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for driving a light emitting apparatuscomprising the steps of; providing the light emitting apparatusincluding a light emitting section for emitting light, the lightemitting section being an LED device which includes an InGaN quantumwell layer as an active layer, and a color of the light of the LEDdevice being blue shifted with a change in value of a driving current;supply a pulse current to the light emitting apparatus to drive thelight emitting apparatus, and controlling separately the peak value andthe duty ratio of the pulse current.
 2. A method according to claim 1,wherein at least the color of the light emitting from the light emittingapparatus is controlled by changing the peak value of the pulse currentsupplied to the light emitting apparatus.
 3. A method according to claim1, wherein the color of the light emitting from the light emittingapparatus is controlled by changing the peak value of the pulse currentsupplied to the light emitting apparatus, and the intensity of the lightfrom the light emitting apparatus is separately controlled by changingthe duty ratio of the pulse current supplied to the light emittingapparatus.
 4. A method for driving a light emitting apparatus accordingto claim 1, wherein; the amount of shifting the color of the lightemitting from the light emitting apparatus is less than 6 nm in thecondition of changing the peak value of the pulse current supplied tothe light emitting apparatus.
 5. A method for driving a light emittingapparatus according to claim 1, wherein; the peak current value iscontrolled so that the emission wavelength of the LED device is changed;and the duty ratio is controlled so that the emission intensity of theLED device is substantially maintained constant.
 6. A method accordingto claim 1, wherein the pulse current has a period equal to or less than30 ms and a pulse width equal to or larger than 0.2 ns.
 7. A method fordriving a light emitting apparatus comprising the steps of; providingthe light emitting apparatus including a light emitting section foremitting light, the light emitting section including an LED device and afluorescent excited by light emitted by the LED device, and a color ofthe light of the LED device being blue shifted with a change in value ofa driving current; supply a pulse current to the light emittingapparatus to drive the light emitting apparatus, and controllingseparately the peak value and the duty ratio of the pulse current.
 8. Amethod according to claim 7, wherein at least the color of the lightemitting from the light emitting apparatus is controlled by changing thepeak value of the pulse current supplied to the light emittingapparatus.
 9. A method according to claim 7, wherein the color of thelight emitting from the light emitting apparatus is controlled bychanging the peak value of the pulse current supplied to the lightemitting apparatus, and the intensity of the light from the lightemitting apparatus is separately controlled by changing the duty ratioof the pulse current supplied to the light emitting apparatus.
 10. Amethod for driving a light emitting apparatus according to claim 7,wherein; the amount of shifting the color of the light emitting from thelight emitting apparatus is less than 6 nm in the condition of changingthe peak value of the pulse current supplied to the light emittingapparatus.
 11. A method for driving a light emitting apparatus accordingto claim 7, wherein; the peak current value is controlled so that theemission wavelength of the LED device is changed; and the duty ratio iscontrolled so that the emission intensity of the LED device issubstantially maintained constant.
 12. A method according to claim 7,wherein, if a value of the driving current is changed, the variation ofa color of light emitting from the light emitting section is larger thanthat of a color of light emitting from the LED device.
 13. A methodaccording to claim 7, wherein the LED device includes an InGaN quantumwell layer as an active layer.
 14. A method according to claim 13,wherein the pulse current has a period equal to or less than 30 ms and apulse width equal to or larger than 0.2 ns.
 15. An apparatus for drivinga light emitting apparatus, comprising; a plurality of the lightemitting apparatus which are disposed so as to form a shape of a plane,and are driven by a driving current so as to obtain a color of the lightbeing blue shifted, the light emitting apparatus including a lightemitting section for emitting light, the light emitting section being anLED device which includes an InGaN quantum well layer as an activelayer, and a color of the light of the LED device being blue shiftedwith a change in value of the driving current; means for supplying apulse current to the light emitting apparatus to drive the lightemitting apparatus, and means for controlling separately the peak valueand the duty ratio of the pulse current, whereby the light emittingapparatus emits light having a desired color by changing the peak valueof the pulse current, even if the driving current is changed.
 16. Amethod according to claim 15, wherein at least the color of the lightemitting from the light emitting apparatus is controlled by changing thepeak value of the pulse current supplied to the light emittingapparatus.
 17. A method according to claim 15, wherein the color of thelight emitting from the light emitting apparatus is controlled bychanging the peak value of the pulse current supplied to the lightemitting apparatus, and the intensity of the light from the lightemitting apparatus is separately controlled by changing the duty ratioof the pulse current supplied to the light emitting apparatus.
 18. Anapparatus for driving a light emitting apparatus, comprising; aplurality of the light emitting apparatus which are disposed so as toform a shape of a plane, and are driven by a driving current so as toobtain a color of the light being blue shifted, the light emittingapparatus including a light emitting section for emitting light, thelight emitting section including an LED device and a fluorescent excitedby light emitted by the LED device, and a color of the light of the LEDdevice being blue shifted with a change in value of the driving current;means for supplying a pulse current to the light emitting apparatus todrive the light emitting apparatus, and means for controlling separatelythe peak value and the duty ratio of the pulse current, whereby thelight emitting apparatus emits light having a desired color by changingthe peak value of the pulse current, even if the driving current ischanged.
 19. A method according to claim 18, wherein at least the colorof the light emitting from the light emitting apparatus is controlled bychanging the peak value of the pulse current supplied to the lightemitting apparatus.
 20. A method according to claim 18, wherein thecolor of the light emitting from the light emitting apparatus iscontrolled by changing the peak value of the pulse current supplied tothe light emitting apparatus, and the intensity of the light from thelight emitting apparatus is separately controlled by changing the dutyratio of the pulse current supplied to the light emitting apparatus.